The naval Battle of Svolder (Svold, Swold) is fought in September 999 or 1000 somewhere in the western Baltic between King Olaf Tryggvason of Norway and an alliance of his enemies.

The backdrop of the battle is the unification of Norway into a single state, long-standing Danish efforts to gain control of the country, and the spread of Christianity in Scandinavia.King Olaf is sailing home after an expedition to Wendland (Pomerania), when he is ambushed by an alliance of Svein Forkbeard, King of Denmark, Olaf Eiríksson, King of Sweden, and Eirik Hákonarson, Jarl of Lade.

Olaf has only 11 warships in the battle against a fleet of at least 70.

His ships are cleared one by one, last of all the Long Serpent, which Jarl Eirik captures as Olaf throws himself into the sea.

After the battle, Norway is ruled by the Jarls of Lade as a fief of Denmark and Sweden.